Solvent extraction of hydrocarbons with n-methyl-2-pyrrolidone



June 24, 1969 H.C. MORRIS ETN- SOLVENT EXTRAC'IIQNV lOF HYDROGARBONS WITH N-METHYL-B-PYRROLIDONE Filed March 13, 1967` Patented June 24, 1969 3,451,925 SOLVENT EXTRACTION F HYDROCARBONS WITH N-METHYL-Z-PYRROLIDONE Herbert C. Morris, Groves, Tex., John I. Nixon, deceased,

late of Bridge City, Tex., by Roberta Lois Nixon, ad-

ministratrix, Bridge City, Tex., assignors to Texaco Inc.,

New York, N.Y., a corporation of Delaware Filed Mar. 13, 1967, Ser. No. 624,970 Int. Cl. Cg 21/20, 21 /02 U.S. Cl. 208-324 4 Claims ABSTRACT 0F THE DISCLOSURE BACKGROUND OP THE INVENTION Field of the invention In the manufacture of lubricating oils from raw petroleum stocks, it is necessary to remove unstable, naturally occurring materials which form deposits or become corrosive in operating equipment as the result of heating and oxidation or both. Additionally, in the case of paraflnic oils, it is often desirable to increase the viscosity index by removing the more aromatic constituents from the oil. To vaccomplish this, it is necessary to remove or destroy a significant amount of material present in the raw stock, typically ranging from 10 to 60 percent depending on the qualities desired in the product oil. The most common way to do this is by extraction with a solvent having selectivity for the more unstable molecules which are predominantly aromatic and non-hydrocarbon materials. Prior to the advent of solvent extraction severe treatment with concentrated sulfuric acid was used to destroy these undesirable materials.

The above processing in which a substantial amount of material is removed from the charge stock is distinctly different from decolorizing treatments in lubricating oil manufacture where only a trace amount of coloring material is removed. Here, handling losses are usually greater than the amount of material removed. Common methods used for color improvement are for example, clay and mild acid treating. Such color improvement ste-ps are often used after solvent refining. A raw stock that is only treated for color improvement is usually unsatisfactory as regards thermal and oxidative stability or viscosity index.

This invention relates to the solvent extraction of hydrocarbon mixtures using N-methyl-Z-pyrrolidone as selective solvent, referred to hereinafter for brevity as NMP. In petroleum refining practice, the various feed stocks and fractions treated are usually produced in contact with steam or water vapor and are stored in contact with liquid water so that they normally contain dissolved water and Aoccasionally small amounts of entrained water. When solvent rening such wet stocks with a solvent such as NMP, the dissolved yand entrained water is taken up by the solvent thereby reducing its solvent power and effectiveness for solvent refining. NMP is also hydroscopic so that it tends to take up water from the surrounding atmosphere in storage and handling. Water, however, has a marked effect upon the solvent power of NMP substantially reducing its ability to dissolve hydrocarbons.

We have now found that water up to 1.0 volume percent may be Ipermitted in the NMP solvent without adversely affecting solvent efciency.

This invention relates to the solvent extraction of hydrocarbon mixtures with a primary solvent comprising NMP containing not more than 1 volume percent water. Separation of NMP from the resulting rainate phase and extract phase is effected by re-extraction with a secondary solvent and then NMP containing less than 1.0 weight percent water is separated from the secondary solvent. Suitable :secondary solvents are materials which exhibit selective solvency for at least one constituent of the raffnate or extract phase. Preferred solvents are, for example, water which selectively dissolves NMP from the extract phase, raffinate phase or both, hydrocarbons of dissimilar boiling range which may be used to selectively dissolve the hydrocarbons extract product from the extract phase. Hydrocarbon solvents having a boiling point at least 50 F. below that of NMP and the hydrocarbon mixture treated are preferred to facilitate their separation from the NMP and extract product by distillation.

Description of the prior art In accordance with prior art practice in the liquidliquid extraction of hydrocarbon mixtures with N-methyl- 2-pyrrolidone, such extraction has been made with dry solvent or with solvent containing more than ve percent by weight of added water to increase selectivity. Separation of the solvent from the resulting extract and rainate phases has been effected by distillation and by water washing.

Summary of the invention In accordance with this invention, a hydrocarbon mixture to be separated into fractions comprising a rainate product of reduced aromaticity and an extract product of increased aromaticity is contacted in -a liquid-liquid extraction zone with a solvent comprising NMP and not more than 1.0 volume percent water. A raiinate phase comprising said ralinate product with a minor portion of NMP dissolved therein and an extract phase comprising said NMP with said extract product dissolved therein are separated.

Solvent is advantageously separated from the rainate product by liquid-liquid extraction of the rallinate phase with water. The resulting solution `of NMP in water is separated by distillation.

Extract product is separated from said extract phase by liquid-liquid extraction with another solvent which is a selective solvent for at least one constituent of said extract phase. The other solvent may be Water or -a hydrocarbon mixture of dissimilar boiling range from the hydrocarbon mixture feed stock and boiling at least 50 F. above or below the NMP solvent. When water is used the resulting solution of NMP in water may be combined with the washings from the reextraction of the rainate phase for separation of the water and NMP. In distilling water-NMP mixtures, the water s separated as distillate and may Ibe reused for additional washing of raffinate phase or extract phase. When recovering NMP from extract phase by reextraction with a lower boiling solvent hydrocarbon, distillation is effected removing the solvent hydrocarbon as distillate from extract product bottoms. The hydrocarbon solvent distillate likewise may be reused in liquid-liquid extraction of the extract phase.

In solvent extraction processes, economic considerations require that solvent losses must be minimized and complexity of recovery operations must be avoided to keep operating costs within acceptable limits. In recovery methods where the solvent is recovered by thermal distillation from the refined oil and extract oil, the initial boiling point of the charge stock must be sufliciently higher than that of the solvent to permit complete separation of the oil and solvent. Also the solvent may be subjected to high temperatures in the heaters used in distillation which may cause deterioration of the solvent and metallurgical problems. Such distillation systems are inherently complicated and expensive to construct.

An object of the present invention is to provide a recovery system simpler in design and operation than those of the prior art. The complete miscibility of NMP with water and the significant reduction of the solvent power of NMP for oil in the presence of small amounts of water may be used to advantage. Most solvents are only partially miscible with water with the result that only limited separation of the solvent from oil can be effected unless large quantities of Water are used. By using water or another secondary solvent lower boiling that NMP for the separation of NMP from extract or raffinate phases it is only necessary to employ temperatures high enough to distill the secondary solvent from the NMP. Furthermore since the solvent need not be distilled from the refined oil and extract products, there is no limitation on the boiling range of the feed stock which may be processed. Accordingly, NMP may be employed for the solvent refining of kerosene and middle distillates, particularly for jet fuel manufacture, which stocks contain constituents coboiling with NMP and which cannot be separated by simple distillation procedures. Also secondary solvents may be selected to permit the use of relatively low temperatures thereby avoiding metallurgical problems and problems of solvent deterioration. Additionally the use of a secondary solvent provides a means to control the Water content of the NMP solvent to achieve highest solvent efficiency.

Brief description of the drawings FIGURE l shows the flow diagram of a solvent refining system employing NMP as solvent with liquid-liquid extraction of both raffinate phase and extract phase with a single secondary solvent which is recovered in common distillation facilities.

FIGURE 2 shows a flow diagram of a solvent refining system employing a second solvent for re-extraction of extract products from NMP in the extract phase and a third solvent for re-extraction of NMP from the raffinate product in the raffinate phase.

Although the figures illustrate particular arrangements of apparatus in which the invention may be practiced, it is not intended to limit the invention to the particular apparatus or materials described.

Referring to FIGURE 1, charge oil enters primary extraction tower through line 1 and is countercurrently contacted therein with recirculated NMP solvent entering through line 4. The charge stock may be any petroleum fraction which is liquid under the treating tower conditions. Treating tower conditions typically include a top tower temperature within the range of about 100 to 390 F. preferably within the range of about 100 to 250 F., and a bottom temperature range of about 100 to 390 F. preferably within the range of about 100 to 200 F. The treating tower pressure is typically maintained within the range of about Oto 100 pounds per square inch gauge and preferably within the range of about 20 to 50 pounds per square inch gauge. An NMP solvent dosage within the range of about 50 to 500 volume percent basis charge is used and preferably within the range of about 100 to 300 volume percent basis charge. Primary extraction tower 20 may be any efficient liquid-liquid contacting device having from 3 to 10 theoretical stages for example packed, spray, baffled or sieve-tray towers with or without mechanical pulse or sonic induced agitation, rotating disc contactors and centrifugal contacting devices. The water content of the entering solvent is maintained at a low level and below a maximum of about 1.0 weight percent basis solvent when treating lubricating oil stocks. The constituents of the charge stock soluble in the solvent are extracted and the resulting extract phase leaves tower 20 through line 3 and comprises principally aromatics dissolved in the bulk of the solvent. The non-extracted constituents of the charge stock comprising principally parains with some dissolved solvent leave tower 20 through line 2. The amount of dissolved solvent in the raffinate phase is characteristically related to the temperature of the tower top and the nature of the refined oil, that is, the raffinate product.

Raffinate phase from tower 20 is discharged through line 2 to raffinate phase extractor 21. In extractor 21 the raffinate phase is countercurrently contacted with Water introduced through line `6. The water is introduced at a dosage of 20 to 100 volume percent basis the NMP in the rafiinate phase and preferably at a dosage within the range of 30 to 50 percent. The contacting temperature is maintained from ambient to about 210 F. and preferably within the range of about to 190 F. Extraction pressures of zero to 150 p.s.i.g. (pounds per square inch gauge) are suitable and pressures within the range of 10 to 2O p.s.i.g. are preferred. Raffinate phase extractor 21 may be any suitable liquid-liquid contacting device as in the case of tower 20. Solvent free raffinate product is discharged through line 5 for further processing or storage not shown. Wash water containing dissolved solvent is discharged from the Ibottom of extractor 21 through line 7.

Extract phase from the bottom of tower 20` is Withdrawn through line 3 and passed to extract phase extractor 23. Extractor 23 may be similar to extractor 21 except that it is necessarily larger in size due to the larger volume of solvent to be removed. Water is introduced at the top of extractor 23 through line 9 at a dosage of about 20 to 100 volume percent basis the solvent in the extract phase and preferably at a dosage of 40 to 50 volume percent. The temperatures and pressures employed are conveniently about the same as those employed in extractor 21. Solvent free extract product is discharged from the top of extractor 23 through line 8 to storage or further use not shown. Water containing dissolved solvent is discharged from the bottom of extractor 23 through line 10, combined with the water-NMP stream in line 7 and passed through line 11 to water-NMP distillation tower 22.

Tower 22 may be a conventional distillation column of 5 to 10 theoretical stages and is operated to separate water as distillate from bottoms comprising NMP and not more than 1.0 weight percent water. The dried solvent is withdrawn from the bottom of tower 22 through line 4 for recycle to primary extraction tower 20. Distillate comprising water is discharged through line 12 and may be used to supply the extraction water passed through lines 6 and 9 to extractors 21 and 23 respectively. Although not shown, it will be understood that makeup water and makeup NMP may be added to the respective circulating systems to compensate for losses, leaks and the like, and correspondingly small amounts may be withdrawn to eliminate contaminants or excess water which may accumulate in the system.

Referring to FIGURE 2, charge stock is introduced through line 30 into the lower portion of primary extractor 50, NMP solvent containing less than 1.0 weight percent water is introduced at the top of extractor 50 through line 31. The charge oil and solvent are countercurrently contacted forming raffinate phase comprising raffinate product with a small amount of NMP dissolved therein. Rafiinate phase is Withdrawn through line 32. Extract phase comprising NMP solvent with extract product dissolved therein is Withdrawn through line 33. Raffinate phase in line 32 is combined with water from line 34 and water phase from line 35 and passed to settler. Advantageously, the amount of water introduced through lines 34 and 35 is sufficient to cause about 80% of the NMP to separate thereby facilitating complete separation of the remaining NMP in subsequent Washing. In

settler S1, an oil phase comprising raffinate product and a reduced amount of dissolved NMP separates from a lower solvent phase containing substantially all of the NMP. The separated oil phaseis passed to countercurrent raffinate phase extractor 52 wherein it is contacted with water introduced through line 44 to remove remaining NMP. Bottoms from extractor 52 comprising water phase with NMP dissolved therein is returned to line 32 as described hereinbefore. Raffinate product is withdrawn from extractor 52 through line 41 and passed to dehydrator 54. Dehydrator 54 may be, for example, a vacuum stripper used to remove dissolved water when the manufacture of a bone dry product is desired. The dried raffinate product is discharged through line 42 to storage or use not shown. Solvent phase from settler 51 is withdrawn through line 43 and passed to water stripping distillation tower 53. In tower 53, water is separated as a distillate through line 44 from bottoms comprising NMP and not more than 1.0 weight percent water is withdrawn through line 31. Water in excess of that passed to extractor 52 is withdrawn through line 45.

Extract phase in line 33 is passed to extract phase extractor 55 wherein it is countercurrently contacted with light hydrocarbon introduced through line 46. The light hydrocarbon in line `46 has a boiling range which does not overlap the boiling range of the charge stock and preferably has a boiling range at least 50 F. below that of NMP. The hydrocarbon solvent may be paraflinic, naphthenic, or aromatic or a mixture of hydrocarbon types, preferably a parafiinic stock is employed. The light hydrocarbon acts as a solvent to dissolve the extract product from the extract phase forming a secondary extract withdrawn through line 47. NMP stripped of the extract product is withdrawn from the bottom of extractor 55 through line 48 and recycled through line 31 to primary extractor 50. Makeup solvent, as required, is added to line 31 from line 29. Extract phase comprising extract product dissolved in hydrocarbon solvent in line 47 is passed to stripper 56 wherein the light hydrocarbon solvent is separated by distillation and returned to extractor 55 through line 46. Extract product is withdrawn from the bottom of stripper 56 through line 49 and discharged to storage or use not shown.

Description of the preferred embodiments The marked effect of water upon the solvent power of NMP is shown in a series of tests made with a stock identified as Wax Distillate 40. This stock is produced by the vacuum distillation of a crude oil and the stock used in this test has a refractive index at 70 C. of 1.4890, a dewaxed viscosity SUS at 210 F. of 89.6, a dewaxed viscosity index of 51.8, and a dewaxed pour point of F. This stock is treated with NMP containing varying amounts of water from 0 to 5.0 percent at the conditions and with the results shown in Table I following:

TABLE I.-NMP REFINING OF WAX DISTILLATE 40 Solvent composition, vol. percent:

Reference to Table I shows that the solvent efficiency of NM'P is significantly reduced by the presence of water at levels of 2.0 and 5.0 volume percent. This is clearly shown by the substantial increase in solvent dosage which is necessary in order to maintain the same refined oil refractive index which is a criterion of the quality of the refined oil. It will be noted that when 2.0 volume percent water is present, about twice as much solvent must be used to maintain he same refractive index, and when 5.0 volume percent water is present, about four times the amount of solvent is necessary to maintain the same refined oil refractive index.

In a second series of tests, a vacuum distillate from crude distillation referred to as a Wax Distillate 7 and having a RI at 70 of 1.4741 is solvent refined with NMP at constant conditions of solvent dosage and extract outlet temperature with varying amounts of water in the NMP solvent. The results of this series of tests are shown in Table II following.

TABLE II.-NMP REFINING OF WAX DISTILLATE 7 Extraction conditions:

Solvent dosage, vol. percent- 200 200 200 200 200 Extraction temperature,

Solvent composition:

NMP, wt. percent 100. 0 99. 5 99. 0 98.0 95. 0

Water, wt. percent 0 0. 5 1. 0 2. 0 5. 0 Yield:

Refined oil, wt. percent.. 76. 0 78. 0 78.5 84.6 89. 4

Extract oil, wt. percent 24. 0 22. 0 21. 5 15. 4 10. 6 Product Tests:

Refined oil RI at 70 C 1.4605 1.4605 1.4608 1.4620 1.4630

Extract RI at 70 C 1. 5030 1. 5072 1.5077 1. 5194 1. 5349 It will be noted that as water is added to the NMP solvent, the amount of oil extracted decreases. The effect of the reduced extraction of undesired constituents however, does not affect refined oil quality until the amount of water in the solvent exceeds about 1.0 percent. The solvents efficiency in producing a refined oil of high quality is then significantly affected as shown by the higher refractive index of the refined oil at solvent water levels above 2.0 Weight percent. It should also be noted that at 0.5 weight percent water, a higher yield of refined oil is obtained having the same quality as the product obtained with water-free solvent as indicated by the refined oil refractive index.

In an embodiment of the invention employing the fiow shown in FIGURE 1, a lubricating oil stock referred to as Wax Distillate 20 is treated with NMP containing a trace of water to produce a refined oil (raffinate product) of increased viscosity index and improved inhibitor response useful in the manufacture of high quality lubricating oil of high viscosity index and high stability. The conditions used and results obtained are shown in Table III following:

TABLE III [NMP refining of wax distillate 20] Treating tower conditions:

Solvent dosage, vol. percent 282 Extract out temp., F 160 Refined oil out temp., F 165 Refined oil yield, vol. percent- 62. 2 Charge oil rate, b.p.d 10, 000

Percent water in NMP, Wt. percent 0.8

Pressure, p.s.i.g 20

Rencd oil Water extraction tower conditions tower Extract tower Temperature, F 150 150 Pressure, p.s.i.g 10 10 Water dosage, w

in charge 50 50 Percent water in oil, wt. percent leaving. 015 02 Percent NMP in oil, wt. percent leaving- 0 0 Water stripper conditions:

Temperature, F., top 290 Temperature, F., bottom 400 Pressure, p.s.i.g 5-10 Percent NMP in water leaving 0.8 Percent water in NMP leaving, wt. percent 0. 0

[Charge stock, wax distillate 20 (contains .02 wt. percent water)] Tests Charge stock Refined oil Extract Gravity, API 25. 3 31. 4 16. 2

Dewaxed Oil VI. 62 97 Dewaxed Pour, +5

RI at 70 C 1. 4792 1 4588 1. 5127 We claim:

1. A process for solvent extraction of a hydrocarbon mixture effecting separating of said hydrocarbon mixture into a rainate product of reduced aromaticity and an extract product of increased aromaticity which comprises:

contacting said hydrocarbon mixture with a first solvent comprising N-methyl-Z-pyrrolidone and within the range of about 0.5 to about 1.0 weight percent water in a first .extraction zone forming a first raffinate phase comprising said raffinate product with a minor portion of said first solvent dissolved therein and a first extract phase comprising said first sol-vent with said extract product dissolved therein,

contacting said first rafiinate phase with a second solvent comprising water in a second extraction zone effecting separation of said first solvent from said first raffinate phase forming a second raffinate comprising said raffinate product and a second extract phase comprising said first solvent dissolved in said second solvent,

passing said second extract phase to a first distillation zone effecting separation of said second solvent as a distillate from distillation bottoms comprising N- methyl-2-pyrrolidone and not more than 1.0 volume percent water,

passing said first distillation bottoms from said first distillation zone to said first extraction zone as at least a portion of said first solvent,

passing said first extract phase to a third extraction zone in contact with a third solvent comprising a liquid hydrocarbon fraction boiling wholly below the boiling range of said hydrocarbon mixture and at least 50 F. below said first solvent effecting formation of a third raffinate comprising said first solvent and a third extract phase comprising said extract product dissolved in said third solvent,

passing said third extract phase to a second distillation zone effecting separation of said third solvent as distillate from bottoms comprising said extract product, and,

passing at least a part of said third raffinate as said first solvent passed to said first extraction zone.

2. The process of claim 1 wherein distillate from said first distillation zone is passed as said second solvent to said second extraction zone.

3. The process of claim 1 wherein distillate from said third distillation zone is passed as said third solvent to said third extraction zone.

4. A process for solvent extraction of a hydrocarbon extract product of increasing aromaticity which com prises:

contacting said hydrocarbon mixture with a first solvent comprising N-methyl-Z-pyrrolidone and within the range of about 0.5 to about 1.0 weight percent water in a first extraction zone forming a first raffinate phase comprising said raffinate product with a minor portion of said first solvent dissolved therein and a first extract phase comprising said first solvent with said extract product dissolved therein,

contacting said first raffinate phase with a second solvent comprising water in a second extraction zone effecting separation of said first solvent from said first raffinate phase forming a second raffinate comprising said raffinate product and a second extract phase comprising said first solvent dissolved in said second solvent,

passing said second extract phase to a .first distillation zone effecting separation of said second solvent as a distillate from distillation bottoms comprising N- methyl-Z-pyrrolidone and not more than 1.0 volume percent water,

passing said first distillation bottoms from said first distillation zone to said first extraction zone as at least a portion of said first solvent,

passing said first extract to a third extraction zone in contact with a third solvent comprising water effecting formation of a third raffinate phase and a third extract phase,

separating the third raffinate product from the raf- 30 Afinate phase and the third extract product from the third extract phase.

References Cited UNITED STATES PATENTS 3l o 2,963,427 12/1960 Nevin 20s- 326 FOREIGN PATENTS 610,414 12/1960 Canada.

HERBERT LEVINE, Primaly Examiner.

U.S. Cl. X.R. 

